You can use these as underarm shields. The pantyliner can be cut to fit. They come in black and white.
I heard this tip on the radio. This would be great for fabrics that stain, or are not washable.
Source: Heard this on a radio show.
This page contains the following solutions.
When ironing a white shirt, sprinkle baby powder under the arms before ironing. This will keep sweat from seeping through the shirt, and avoid stains.
This is a page about deodorant that won't stain clothing. Unfortunately sometimes the deodorant we use can stain our clothing.